The invention relates to “active implantable medical devices” as defined by Directive 90/385/EEC of 20 Jun. 1990 of the Council of the European Communities, specifically implants for delivering therapies of stimulation of the vagus nerve, called VNS therapies (Vagus Nerve Stimulation). The stimulation of the nervous system is recognized as a possibility for treating many disorders such as epilepsy therapy, pain, heart failure, sleep apnea, obesity, or others.
The devices used for this purpose typically include a lead equipped with an electrode implanted on the vagus nerve and a generator supplying VNS pulses on this electrode. If the vagus nerve is to be stimulated further in synchronism with the heartbeat, the apparatus may further include one or more cardiac leads for collecting of depolarization waves of the myocardium.
In some devices, the VNS therapy is to add an arbitrary signal (VNS pulses) overlapping signals naturally conveyed by the nervous system. This stimulation of the vagus nerve may act in an efferent manner, carrying the pulses to the organ, or act in an afferent manner (e.g., pulses are sent to the brain to affect the central nervous system). The arbitrary signal including VNS pulses may be interpreted by the central nervous system as a solicitation that the central nervous system will try to compensate to oppose, and thereby preventing production of the expected effects.
The profile of stimulation of the vagus nerve consists of bursts (or trains) of repetitive pulses produced during a period known as “activity”, followed by an “inactive” period during which the stimulus is no longer delivered. The energy of each burst is determined by the width, amplitude, number, frequency, and pulse morphology, parameters that all are adjusted by appropriate setting of the VNS pulse generator.
The problem addressed by the invention is related to the fact that if neuronal therapy by VNS stimulation is effective at the beginning of its application, the VNS stimulation often seems to lose its effectiveness over time (e.g., a few weeks) due to compensation phenomena coming from the creation of a loop of physiological control.